Catalytic cracking apparatuses are crucial means for crude oil refining, and the economic benefits of refineries depend on the overall product distribution of these apparatuses. Recently, as the growing trend towards crude oils having higher density and poorer quality, a higher heavy-oil-conversion capacity and higher selectivity for valuable products are demanded for FCC catalysts. The cracking activity and activity stability of the Y-type molecular sieve, a major provider of the cracking activity of heavy-oil cracking catalysts, are key factors in determining the heavy-oil-conversion capacity of FCC catalysts.
Therefore, in order to improve the cracking activity and activity stability of Y-type molecular sieves, extensive investigations have been carried out in domestic and abroad research institutes. Currently, it is largely agreed that the framework structural stability and the activity stability of molecular sieves can be improved by localizing as many rare earth ions as possible in sodalite cages in the process of rare earth modification of molecular sieves so as to suppress dealumination of the molecular sieve framework during steam aging. Chinese Patent No. ZL200410058089.3 describes a method for preparing rare earth-modified Y-type molecular sieves, comprising steps of adjusting the pH of the system to 8-11 using an alkali solution after completion of the rare earth exchange reaction, and then carrying out conventional subsequent treatment processes. In the molecular sieves prepared by this method, rare earth ions are completely located in small cages (sodalite cages). Chinese Patent No. ZL200410058090.6 describes the reaction performance of the molecular sieves described in ZL200410058089.3, wherein the catalyst reaction results show that localization of rare earth metals in sodalite cages improves the structural stability and the activity stability of the molecular sieves, manifested in that the heavy-oil-conversion capacity of the catalyst is greatly improved; however, this catalyst has poor coke selectivity.
Chinese patent No. ZL97122039.5 describes a preparation method of ultrastable Y-type zeolites, comprising steps of contacting a Y-type zeolite with an acid solution and an ammonium-containing solution, and subjecting them to a high-temperature steam treatment, wherein the amount of the acid used is 1.5 to 6 moles of hydrogen ions per mole of framework aluminum, the concentration of the acid solution is 0.1 to 5 N/L, the Y-type zeolite is kept in contact with the acid solution at a temperature of 5 to 100° C. for a duration of 0.5 to 72 h, and the weight ratio between the Y-type zeolite and the ammonium ion is 2 to 20. The modification method involved in above patent requires addition of an ammonium-containing solution for the purpose of lowering the sodium oxide content in the molecular sieve or reducing the damage to the molecular sieve structure caused by acidic gases during calcination. Although FCC catalysts made from this molecular sieve are characterized by a high heavy-oil-conversion capacity and a high yield of light oils, this molecular sieve modification technique has the following technical disadvantages: 1) since a large number of ammonium ions are added in the preparation process, ammonium-containing ions would eventually enter the atmosphere or waste water, increasing ammonia nitrogen pollution and the cost for pollution control; 2) the method of above patent is unable to effectively solve the issue of particle agglomeration in a molecular sieve, which issue reduces specific surface area and pore volume of the molecular sieve and increases the obstruction in the pore channel during exchange in the molecular sieve, making it difficult to accurately and quantitatively localize the element for modification in the cages of the molecular sieve; 3) moreover, in above patent it is further mentioned that rare earth ions may also be introduced by ion exchange, during or after the contact between the Y-type zeolite and the ammonium-containing solution, and that during the ion exchange, ammonium ions compete with rare earth ions and preferentially take up the positions intended for rare earth ions, thereby hindering rare earth ions from entering the cages of the molecular sieve by exchange, and also lowering the utilization of rare earth ions.
Chinese patent No. ZL02103909.7 describes a method for preparing rare earth-containing ultrastable Y-type molecular sieves by subjecting a NaY molecular sieve to one exchange process and one calcination process, characterized in that the NaY molecular sieve is placed in an ammonium-containing solution and subjected to chemical dealumination by a chemical dealumination chelating agent containing oxalic acid and/or oxalate salts at 25 to 100° C. for 0.5 to 5 h, a rare earth solution is then introduced under stirring to produce a rare earth precipitate that contains rare earth oxalate, and the precipitate is filtered and water-washed to give a filter cake, followed by a hydrothermal treatment to afford the molecular sieve product. Although the molecular sieve prepared by this method has certain resistance to vanadium contamination, it has relatively low activity stability and cracking activity, and is insufficient to meet the requirement set out by the growing trend towards crude oils having higher density and poorer quality. This is mainly attributed to the position distribution of rare earth ions in the super-cages and sodalite cages of the molecular sieve during its modification. This method demonstrates that rare earth ions are present in the molecular sieve system in two forms, i.e., a part of the rare earth enters sodalite cages in an ionic form, while the other part is dispersed on the surface of the molecular sieve as an independent phase of rare earth oxide (the precursor of which is rare earth oxalate and is converted into rare earth oxide after subsequent calcination), which reduces the stabilizing and supporting effect of rare earth ions on the molecular sieve structure. Furthermore, this method also poses a remarkable problem of ammonium nitrogen pollution, and the oxalic acid or oxalate salts added are also toxic and detrimental to the environment and human.
CN200110029875.0 discloses a preparation method of a rare earth ultrastable Y-type zeolite, characterized in that this method includes a step of treating zeolite with a mixed solution of a rare earth salt and citric acid or with a mixed solution of an inorganic ammonium salt, a rare earth salt and citric acid. This method can simplify the process, and the zeolite prepared thereby, when serving as an active component of a cracking catalyst, is advantageous in lowering the olefin content in the catalytically cracked gasoline products, and substantially increasing the yield of the catalytically cracked light oil products. However, this method does not specify the location of rare earth ions in the molecular sieve.